Avoid the use of malloc() and free().

[bangerth]

In structured_data.cc, we do

        char *filename = static_cast<char *>(std::malloc (maxsize * sizeof(char)));
        int ret = std::snprintf (filename,
                                 maxsize,
                                 filename_and_path.c_str(),
                                 boundary_name.c_str(),
                                 filenumber);

        AssertThrow(ret >= 0, ExcMessage("Invalid string placeholder in filename detected."));
        AssertThrow(ret< maxsize, ExcInternalError("snprintf string overflow detected."));
        const std::string str_result (filename);
        std::free (filename);

We should avoid the use of malloc() and free() and instead replace the explicit memory allocation via the use of a std::unique_ptr and std::make_unique().